1. Field of the Invention
The present invention relates to a method of designing a high-frequency circuit such as a high-frequency linear amplifier for signals of high power level.
2. Description of Related Art
A method previously employed to design a high-frequency linear amplifier for high power level (usually -30 dBm or higher) signal, with a field-effect transistor (FET) is as follows.
Once the amplifier specification is determined, an FET that matches the specification is selected, and its ID-VD (drain current--drain voltage) characteristics, S-parameters, etc. are measured. Then, the values that match the results of the measurement are inputted to an FET model provided in a nonlinear circuit simulator. An input matching circuit and an output matching circuit are designed so that the desired characteristics such as gain, maximum linear power, saturation power, etc. can be obtained.
Among the commercially available nonlinear circuit simulators are the "MDS" manufactured by Hewlett Packard and "Microwave Harmonica" by Compact Software.
However, FET models provided in nonlinear circuit simulators do not have sufficient accuracy that can faithfully reproduce the actually measured values, for example, in low drain voltage regions and deep gate bias regions. As a result, models that can only match the actually measured values at some of the bias points have to be used. This has led to the problem that circuit design with good accuracy cannot be made. Furthermore, when using such models, such parameters as mutual conductance, gate-source capacitance, etc. have to be set to match the measurement results. The problem here is that selecting optimum values for these parameters takes considerable time.